The use of digital pulse shaping filters in digital communication systems is well known. Co-pending patent application Ser. No. 09/302,078 describes a novel digital pulse shaping filter that can be employed in a wireless communication system. Other pulse shaping filters are known in the art, including Nyquist filters, raised cosine filters, and the like. The teachings of the present invention apply to all such digital filters.
Various techniques for synchronizing a received signal in order to determine the information bearing point of the signal are also known. A novel approach to signal synchronization is provided in co-pending patent application Ser. No. 09/561,706. As taught in that application, a receiving device can synchronize to an incoming signal by taking advantage of a property of pulse shaping filters, specifically that the relative energy distribution of the output waveform with respect to the information bearing point of the signal is determinable. In one preferred embodiment of that application, the magnitude of the signal coming out of the pulse shaping filter is measured (over time) and from the measurement of the magnitude—which is related to the energy distribution of the signal—the information bearing point is determined and synchronized to.
The above described approach provides significant information throughput efficiency and the ability to synchronize on a signal without the need for pilot symbols or synchronizing tones. The improvements come at a cost of extensive signal processing requirements, however. This is because—even though there is only one information bearing point for each transmitted symbol—the incoming signal is sampled at a much higher rate than the symbol rate in order to ensure that at least one sample is at or very near the information bearing point. As such, each sample must be digitally processed, even though only one sample (or sometimes two or three in the case of interpolation) is ultimately useful for decoding the signal.